diff --git a/trytond/model/fields/char.py b/trytond/model/fields/char.py
index 301c4c8ed5..97374f3ca2 100644
--- a/trytond/model/fields/char.py
+++ b/trytond/model/fields/char.py
@@ -82,5 +82,6 @@ class Char(FieldTranslate):
     def definition(self, model, language):
         definition = super().definition(model, language)
         definition['autocomplete'] = list(self.autocomplete)
-        definition['size'] = self.size
+        if self.size is not None:
+            definition['size'] = self.size
         return definition
diff --git a/trytond/model/fields/many2many.py b/trytond/model/fields/many2many.py
index 4c2d5621f2..e9a5ab6648 100644
--- a/trytond/model/fields/many2many.py
+++ b/trytond/model/fields/many2many.py
@@ -451,5 +451,6 @@ class Many2Many(Field):
         definition['search_context'] = encoder.encode(self.search_context)
         definition['search_order'] = encoder.encode(self.search_order)
         definition['sortable'] &= hasattr(model, 'order_' + self.name)
-        definition['size'] = encoder.encode(self.size)
+        if self.size is not None:
+            definition['size'] = encoder.encode(self.size)
         return definition
diff --git a/trytond/model/fields/one2many.py b/trytond/model/fields/one2many.py
index 3dc41fe7cc..ef2dc38227 100644
--- a/trytond/model/fields/one2many.py
+++ b/trytond/model/fields/one2many.py
@@ -342,6 +342,7 @@ class One2Many(Field):
             definition['relation_field'] = self.field
         definition['search_context'] = encoder.encode(self.search_context)
         definition['search_order'] = encoder.encode(self.search_order)
-        definition['size'] = encoder.encode(self.size)
+        if self.size is not None:
+            definition['size'] = encoder.encode(self.size)
         definition['sortable'] &= hasattr(model, 'order_' + self.name)
         return definition